Field of the Invention
The present invention relates to a torque wrench in which a clamping torque of a main spindle can be controlled, and more particularly to an impact wrench or oil pulse wrench comprising an air motor unit having a driving shaft, an oil pressure pulse generating unit for converting a driving torque of the driving shaft into a pulsatory torque, and a main spindle which is rotated in a pulsatory manner by means of the pulsatory torque generated by the oil pulse generating unit.
There have been proposed various types of the above mentioned impact wrench with the torque controlling faculty. In such an impact wrench, a torque applied to a main spindle is detected and when the detected torque exceeds a predetermined threshold value, a shut-off valve provided in an air conduit for supplying a compressed air to an air motor is closed to stop the rotation of the main spindle. For instance, in Japanese Utility Model Application Laid-open Publication Jikkai Sho 61-81878, there is described a known impact wrench with the torque controlling faculty, in which a planetary reduction gear is arranged between an output shaft of an oil pressure pulse generating unit and a main spindle to which a tool is detachably secured and a ring gear is formed in an inner wall of a gear casing of the planetary reduction gear such that the ring gear is engaged with the planetary reduction gear. A strain gauge is applied on an outer surface of the gear casing and a clamping torque applied to the main spindle is measured by processing an output signal of the strain gauge.
In this known impact wrench, since use is made of the planetary gear mechanism, it is impossible to detect the torque applied to the main spindle directly. Further, an operator of the impact wrench is subjected to a rather large repelling force of the gear casing.
In Japanese Patent Application Laid-open Publication Kokai Sho 61-4676, there is described another known impact wrench having the torque controlling faculty, in which a strain gauge is directly applied on a main spindle and an output signal of the strain gauge is taken out by means of a rotary transformer whose primary coil and secondary coil are provided on the main spindle and a main housing of the impact wrench, respectively.
In this known impact wrench, the torque of the main spindle can be directly detected without making contact with the main shaft, so that the repelling force applied to the operator can be made small, but the strain gauge and the primary coil of the rotary transformer are secured to the main spindle which is rotated at a very high speed, so that they are liable to be pealed off the main spindle. Therefore, the fluctuation of the detected torque is very large and the accuracy of the torque measurement might be reduced within a relatively short time period.